A wide range of medical image techniques is available for the diagnosis and treatment of diseases. One can differentiate between morphological imaging, such as in Computed Tomography (CT), Magnetic Resonance Imaging (MRI), X-ray, Ultrasound, etc. or functional imaging as in Gamma Ray Radiography, Single-Photon Emission Computed Tomography (SPECT) or Positron Emission Tomography (PET).
Nuclear medicine is a special branch of medicine where physiological processes can be imaged by detecting the radiation from radioactive tracing substances injected into regions of interest (such as organs, bones, tissue, etc.) of the object under examination.
In the case of PET, when the decaying radioactive tracer emits a positron, the positron annihilates with an electron creating a pair of high-energy gamma ray photons which are emitted in opposite directions. To detect the produced gamma rays, in general scintillation crystal detectors are used in the following way.
In the case of PET, when a gamma ray photon enters the crystal it interacts with the atoms of the crystal, creating a flash of isotropically emitted lower energy photons following the excitation or ionization of the crystal atoms. In this so called scintillation event the energy of the gamma ray photons is thus transformed into lower energy (usually visible range) photons, which then can be measured by photo sensors. Furthermore the energy of the lower energy photons is hereby proportional to the energy of the incident gamma ray photons.
When two gamma ray photon detections at opposite locations are made at the same or almost the same time (within a tolerance of a few nano seconds, due to different photon travel distances to the detectors) they are assumed to have been created in the same annihilation process. It is then known that the origin of this gamma ray pair emission must lie on the line connecting the two detection positions. This line is commonly referred to as line of response (LOR, see also US2010/0044571A1). The cross section of a plurality of gamma ray pair detections and LORs can then be used to create a three dimensional map of the region of interest, where the concentration of the radioactive tracers is the highest.
The photo sensors used to measure the scintillation photons are normally position sensitive photomultiplier tubes (PMT) or the recently proposed semiconductor based detectors such as silicon multipliers (SiPM). The SiPMs usually consist of an array of avalanche photo diodes operating in Geiger mode. While SiPMs need less supply voltages than PMTs they suffer from thermal background noise, which increases proportionally to the square root of the covered detector surface area. It is therefore desirable to keep the active SiPM detector area as small as possible, but without reducing the light emitting area of the scintillator.
As mentioned above the energy distribution measured by the photo sensor of such gamma ray cameras is proportional to the energy distribution of the incident gamma ray and so allows to discriminate between the energy of the original incident gamma radiation and secondary radiation such as background radiation or Compton scattered events. Furthermore the detected scintillation light contains information on the spatial location of the scintillation event inside the scintillator and hence on the source of the gamma radiation, the region of interest to be imaged.
Prior art documents U.S. Pat. Nos. 4,150,292 and 6,858,847 are describing embodiments of such detectors in the field of Nuclear Imaging.
It is known that scintillator based gamma ray detectors have a poorer energy resolution than solid state high purity Germanium detectors. However scintillation detectors are still the most common type of detectors used due to their moderate costs, their high efficiency, applicability on large scales and the possibility to operate at room temperatures.
In recent years considerably effort has been put into improving the energy resolution of scintillation detectors by maximizing the collection efficiency of the scintillation light created following the absorption of the gamma ray photons inside the scintillator. Computer simulations and experiments have shown that the ability to capture this light is strongly influenced by the geometry of the scintillator, the coating material of its outer surface and the scintillator-photo detector coupling.
For example a diffuse reflecting optical finish increases the amount of light reaching the photo sensor, whereas using only a specular reflector (i.e. a polished surface) has the opposite effect. The best materials to wrap in light efficient scintillators have a high diffusive reflectivity. Polytetraflouroethylene (PTFE, Teflon) for example is a common material used to coat the outer surface of a scintillator. Recent work US 2011/0017916 A1 shows how to combine a diffuse and specular reflective layer to increase light collection efficiency, however at the expense of worsening the information on the original distribution of the scintillation light. In US 2011/0017916 A1 the objective is to collect all light, regardless of its spatial origin, thereby loosing spatial resolution.
In US 2010/0044571 A1 a photo sensor is mounted onto the scintillator surface of entry of the gamma ray to increase spatial resolution, exploiting the fact that most scintillation events are occurring close to the surface of entry due to the exponential interaction probability. However, such a photo-sensor-on-entrance-surface configuration has the disadvantage that the gamma radiation is attenuated when traversing the photo sensor, which can lead to significant losses at lower energies.
The light distribution of a scintillation event measured by the position sensitive photo sensor, i.e. scintillation light intensity as a function of position on photo sensor depends on the three-dimensional position of the scintillation event inside the scintillator. From the centroid position, the width and higher moments of the light distribution measured by the photo sensor, the original location of the scintillation event can be reconstructed, as shown in EP1617237A1. The accuracy of how well the location of the scintillation event can be determined depends on the dispersion of the scintillation light before reaching the position sensitive photo sensor. In a state-of-the-art scintillator reflections at the scintillator faces increase the light collection efficiency but also lead to a broadening, respectively blurring of the light distribution received by the photo sensor, resulting in less accurate estimates of the three-dimensional scintillation event position, and therefore in a loss of spatial resolution of the gamma ray detector.
Current means of improving the spatial resolution within the field of view of the gamma ray camera commonly only relate to the use of different types of collimators mounted in front of the gamma ray detector in order to filter the gamma rays before the occurrence of a scintillation event (Kimiaei et al., 1996, Journal of Nuclear Medicine, Vol. 37. No. 8, p 1417-1421).